1. Field of the Invention
This invention relates to a temperature and humidity compensating device for preventing deterioration in image formation from occurring due to low ambient temperatures and high humidities in an apparatus for forming electrostatic latent images on the surface of a photosensitive medium having a photoconductive layer and a conductive layer.
2. Description of the Prior Art
Description will hereinafter be made by taking, as an example, an electrophotographic copying apparatus in which a photosensitive medium is exposed to a light image and a corona discharge to form thereon an electrostatic latent image which is in turn developed into a visible image.
In copying machines wherein a photosensitive medium having a photoconductive material is subjected to steps of charging, exposure development, etc., the important factors for providing a good quality image are the characteristics of the photosensitive medium, the corona discharge and the developer.
Electrophotographic copying machines are usually used in an environment which may contain one of various combinations of temperature and humidity, such as low temperature and high humidity, or high temperature and high humidity. Among these ambient conditions, low humidity offers no inconvenience to the sensitivity of the photosensitive medium and the corona discharger. However, in an environment containing either low temperature or high humidity, the following problems will be encountered.
At low temperatures below the normal temperature range of 15.degree. to 20 C., especially below 10.degree. C., the electrostatic latent image resulting from application of a light image to a photosensitive medium has its contrast decreased due to the properties of the electrons trapped in the photoconductive layer of the photosensitive medium. Further, the corona discharge current is decreased to reduce the discharging efficiency, thus further decreasing the contrast of the image. Also, where image development is carried out by the use of a liquid as a carrier, low temperature may induce a poor developing effect because a falling temperature reduces the mobility of the toner particles in the developing liquid. The result is a low contrast in the latent image which in turn leads to difficulties in providing the best quality of visible image.
If left under high humidity conditions for a long time, the photosensitive medium will absorb moisture and the resistivity thereof will thus be decreased. Such decreased resistivity, coupled with the well-known fact that the discharge current resulting from corona discharge is decreased at high humidity, decreases the contrast of the latent image and cannot promise a good quality of image. This will be apparent, for example, from the fact that, in the wintertime of a cold northern district, a copying machine adjusted to normal room temperatures of 20.degree. to 25.degree. C. cools down to the order of -10.degree. C. at night (when the heating equipment in offices are turned off), and in the morning when the heating equipment is turned on, it takes much time for the interior temperature of the cold machine to rise up to substantially the same level as the room temperature and, in addition, dew drops are created on the surface of the photosensitive medium so that image reproduction of good quality is unobtainable during the early part of the office hours.
As a countermeasure for low ambient temperatures, a preheater has heretofore been provided in copying machines to prevent the interior temperature of the machine from falling below a certain level.
Nevertheless, there has been no countermeasure for protecting the performance of the copying machine not only against low ambient temperatures but also against an environment containing either or both of low temperature and high humidity, much less a countermeasure which can economically and safely sustain a subject to be minimally protected against the adverse ambient conditions.